Many methods and apparatus exist for generating an image on a record medium such as for example applying heated electrodes to temperature sensitive paper, projecting light in a desired shape on light sensitive paper, or applying a charge pattern to a paper designed to retain such charge. These techniques have inherent problems when selectively addressable symbols enclosed by other symbols are involved.
For example, U.S. Pat. No. 3,736,406, issued May 29, 1973 to Vossen et al. and assigned to the common assignee, illustrates a thermographic print head with selectable (heatable) concentric circular electrodes on an electrically insulated substrate, and a heat sensitive paper. An inner circle is selected by applying power thereto over conductors which, of necessity, cross outer circular electrodes. The conductors may be designed to produce no heat in and of themselves but have been found in practice to undesirably conduct heat away from any heated electrodes over which they pass, creating voids in the circular rings appearing on the heat sensitive paper. If the conductors are heated to overcome the problem of voids they then create undesired radial lines. Similarly with photographic techniques some sort of mask must be provided which has inner addressable segments, the addressing means for which must cross outer segments without themselves causing undesired marks to appear on the record medium. Likewise with apparatus for applying a charge pattern there is a problem in applying to an inner symbol enclosed by an outer closed symbol without undesirably altering the charge pattern on the outer symbol.